1. Field of the Invention
The present invention relates to the separation of enantiomers, i.e., those isomers in which the arrangement of atoms or groups is such that the two molecules are not superimposable. The invention more particularly relates to a high performance chiral selector useful, for example, as a chiral stationary phase (CSP) in liquid chromatographic separation of enantiomers.
2. Description of the Prior Art
Stereoisomers are those molecules which differ from each other only in the way their atoms are oriented in space. Stereoisomers are generally classified as diastereomers or enantiomers; the latter embracing those which are mirror-images of each other, the former being those which are not. The particular arrangement of atoms that characterize a particular stereoisomer is known as its optical configuration, specified by known sequencing rules as, for example, either + or - (also D or L) and/or R or S.
Though differing only in orientation, the practical effects of stereoisomerism are important. For example, the biological and pharmaceutical activities of many compounds are strongly influenced by the particular configuration involved. Indeed, many compounds are only of widespread utility when employed in a given stereoisomeric form.
Living organisms usually produce only one enantiomer of a pair. Thus only (-)-2-methyl-1-butanol is formed in yeast fermentation of starches; only (+)-lactic acid is formed in the contraction of muscle; fruit juices contain only (-)-malic acid, and only (-)-quinine is obtained from the cinchona tree. In biological systems, stereochemical specificity is the rule rather than the exception, since the catalytic enzymes, which are so important in such systems, are optically active. For example, the sugar (+)-glucose plays an important role in animal metabolism and is the basic raw material in the fermentation industry; however, its optical counterpart, or antipode, (-)-glucose, is neither metabolized by animals nor fermented by yeasts. Other examples in this regard include the mold Penicillium glaucum, which will only consume the (+)-enantiomer of the enantiomeric mixture of tartaric acid, leaving the (-)-enantiomer intact. Also, only one stereoisomer of chloromycetin is an antibiotic; and (+)-ephedrine not only does not have any drug activity, but it interferes with the drug activity of its antipode. Finally, in the world of essences, the enantiomer (-)-carvone provides oil of spearmint with its distinctive odor, while its optical counterpart (+)-carvone provides the essence of caraway.
Accordingly, it is desirable and oftentimes essential to separate stereoisomers in order to obtain the useful version of a compound that is optically active.
Separation in this regard is generally not a problem when diastereomers are involved: diastereomers have different physical properties, such as melting points, boiling points, solubilities in a given solvent, densities, refractive indices etc. Hence, diastereomers are normally separated from one another by conventional methods, such as fractional distillation, fractional crystallization or chromatography.
Enantiomers, on the other hand, present a special problem because their physical properties are identical. Thus they cannot as a rule--and especially so when in the form of a racemic mixture--be separated by ordinary methods: not by fractional distillation, because their boiling points are identical; not by conventional crystallization because (unless the solvent is optically active) their solubilities are identical; not by conventional chromatography because (unless the adsorbent is optically active) they are held equally onto the adsorbent. The problem of separating enantiomers is further exacerbated by the fact that conventional synthetic techniques almost always produce a mixture of enantiomers. When a mixture comprises equal amounts of enantiomers having opposite optical configurations, it is called a racemate; separation of a racemate into its respective enantiomers is generally known as a resolution, and is a process of considerable importance.
Various techniques for separating enantiomers are known. Most, however, are directed to small, analytical quantities, meaning that other drawbacks aside, when applied to preparative scale amounts (the milligram to kilogram range) a loss of resolution occurs. Hand separation, the oldest method of resolution, is not only impractical but can almost never be used since racemates seldom form mixtures of crystals recognizable as mirror images.
Another method, known as indirect separation, involves the conversion of a mixture of enantiomers--the racemate--into a mixture of diastereomers. The conversion is accomplished by reacting the enantiomers with an optically pure derivatizing agent. The resultant diastereomers are then separated from one another by taking advantage of their different physical properties. Once separated by, for example, fractional crystallization, or more commonly, chromatography, the diastereomers are re-converted back into the corresponding enantiomers, which are now optically pure. Though achieving the requisite separation, the indirect method suffers in that it is time consuming and can require large quantities of optically pure derivatizing agent which can be expensive and is oftentimes not recoverable. Moreover, the de-derivatizing step may itself result in racemization thus defeating the purpose of the separation earlier achieved.
A more current method that avoids some of the drawbacks attendant the indirect method is known as the direct method of separation. The direct method, much like the indirect method, involves the formation of a diastereomeric species. However, unlike the indirect method, this species is transient, with the stability of one species differing from the other.
In one application of the direct method, as disclosed, e.g., in copending and commonly assigned U.S. patent application Ser. No. 528,007, filed May 23, 1990, now U.S. Pat. No. 5,080,795 the contents of which are incorporated herein by reference, enantiomers of compounds such as amino acids, amino esters, alcohols, amines, sulfonic acid or derivatives thereof are separated by means of a liquid membrane that contains a chiral carrier, such as the derivatized amino acid (S)-N-(1-naphthyl)leucine octadecyl ester. The chiral carrier is capable of forming a stable complex with one of the enantiomeric configurations. The liquid membrane is located on one side of a semi-permeable barrier and the mixture of enantiomers is located on the other side of the barrier. The liquid membrane containing the chiral carrier impregnates the semi-permeable barrier under conditions effective to permit or cause a stable complex between the chiral carrier and one of the enantiomeric configurations to form in the barrier. The liquid membrane containing the stable complex is passed to a second location where the conditions are effective to dissociate the stable complex, thus allowing the recovery of the complex-forming enantiomer to take place. In one embodiment of this application, a hollow membrane fiber membrane is employed as the semi-permeable barrier.
In another, more common application of the direct method, the mixture of enantiomers is allowed to interact with a chiral stationary phase as resides, e.g., in a chromatographic column. The enantiomer that interacts more strongly with the chiral stationary phase will have a longer residence time in the column; hence, a separation of enantiomers will occur. Further, when the mode of interaction with the chiral stationary phase can be characterized, the elution order can be predicted.
Examples of chiral stationary phases include those based upon (L)-N-(3,5-dinitrobenzoyl)leucine, which is useful in separating enantiomers of N-aryl derivatized amino acids and esters, and those based upon (L)-N-(1-naphthyl)leucine which has been effectively used to separate N-(3,5-dinitrobenzoyl) derivatized amino compounds. High performance liquid chromatographic (HPLC) columns packed with silicabonded CSP's of a variety of .pi.-electron acceptors and .pi.-electron donors--including derivatives of phenylglycine, leucine, naphthylalaninc and naphthylleucine are commercially available from Regis Chemical Company, Morton Grove, Ill.
Other examples of chiral stationary phases used in the direct separation of enantiomers include, e.g., that based upon N-(3,5-dinitrobenzoyl)-.alpha.-amino-2,2-dimethyl-4-pentenyl phosphonate, as particularly described in commonly assigned U.S. patent application Ser. No. 761,212, filed on Sep. 17, 1991, now U.S. Pat. No. 5,254,258, which is useful in separating enantiomers of .beta.-amino alcohol compounds, such as .beta.-blockers; and that based upon 4-(3,5-dinitrobenzoyl)amino-3-(undec-10-enyl)-1,2,3,4-tetrahydrophrenanthr ene, as particular described in copending and commonly assigned U.S. patent application Ser. No. 763,043, filed Sep. 20, 1991, now abandoned, which is useful in separating enantiomers of non-steroidal anti-inflammatory agents, such as naproxen.
While these efforts indicate that there is a wide variety of useful chiral selectors available, there nevertheless continues to be a pressing need for chiral selectors having analytical and, importantly, preparative scale applicability over a broad range of enantiomeric compounds, especially chiral selectors that evince decreased retention and increased selectivity so as to provide improved qualitative separations of these compounds.